1. Field of the Invention
The present invention relates to a polytetrafluoroethylene-based-resin tube and a method of producing the same.
2. Description of the Prior Art
Tubes made of a fluororesin are preferred from the lubricity and chemical resistance viewpoint, in particular. In certain fields of application, such tubes are required to have a thin wall.
Those fluororesin-made thin-wall tubes which are disclosed in JP Kokai (Laid-open Japanese Patent Application) H08-168521, for instance, are the products obtained by molding, by the melt extrusion technique, a mixture composed of a tetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymer and 5-25% by weight of polytetrafluoroethylene [PTFE].
In molding PTFE, which shows a very high melt viscosity, into tubes, it is thinkable to employ the paste extrusion molding technique in lieu of the melt extrusion molding technique. However, the paste extrusion molding technique has a problem in that the wall thickness can be reduced only to a limited extent by that technique.
ASTM D 3295-90, which is concerned with PTFE-made tubes, describes such tubes with an inside diameter of 0.25 mm and a wall thickness of 0.1 mm as ones having the smallest circumference and thinnest wall thickness. However, in some fields of application where extremely thin tubes are required, there is a problem, namely the reduction in wall thickness is still unsatisfactory.
Those thin-wall PTFE tubes which are obtainable by applying a PTFE dispersion onto a core wire, such as a copper wire, sintering the same and, after the formation of an outer resin layer, drawing out the core wire have been proposed (cf. e.g. JP Kokai 2000-316977 and JP Kokai 2000-51365). However, they have a problem, namely they are low in tensile strength.
PTFE-made tubes are desired to be improved in the tensile strength in the longitudinal direction. In particular when they have a thin wall, the tensile strength in the longitudinal direction generally tends to be inferior.
In view of the above-discussed state of the art, it is an object of the present invention to provide a polytetrafluoroethylene-based-resin tube high in longitudinal tensile strength and a method of producing the same.